The present invention relates to industrial controllers used for real-time control of industrial processes, and in particular to high-reliability industrial controllers appropriate for use in devices intended to protect human life and health. “High reliability” refers generally to systems that guard against the propagation of erroneous data or signals by detecting error or fault conditions and signaling their occurrence and/or entering into a predetermined fault state. High reliability systems may be distinguished from high availability systems, however, the present invention may be useful in both such systems and therefore, as used herein, high reliability should not be considered to exclude high availability systems.
Industrial controllers are special purpose computers used in controlling industrial processes. Under the direction of a stored control program, an industrial controller examines a series of inputs reflecting the status of the controlled process and changes a series of outputs controlling the industrial process. The inputs and outputs may be binary, that is, on or off, or analog, providing a value within a continuous range. The inputs may be obtained from sensors attached to the controlled equipment and the outputs may be signals to actuators on the controlled equipment.
“Safety systems” are systems intended to ensure the safety of humans working in the environment of an industrial process. Such systems may include the electronics associated with emergency stop buttons, interlock switches and machine lockouts. Traditionally, safety systems have been implemented by a set of circuits wholly separate from the industrial control system used to control the industrial process with which the safety system is associated. Such safety systems are “hard-wired” from switches and relays, some of which may be specialized “safety relays” allowing comparison of redundant signals and providing internal checking of conditions such as welded or stuck contacts. Safety systems may use switches with dual contacts providing an early indication of contact failure, and multiple contacts may be wired to actuators so that the actuators are energized only if multiple contacts close.
Hard-wired safety systems have proven inadequate, as the complexity of industrial processes has increased. This is in part because of the cost of installing and wiring relays and in part because of the difficulty of troubleshooting and maintaining the “program” implemented by the safety system in which the logic can only be changed by rewiring physical relays and switches.
For this reason, there is considerable interest in implementing safety systems using industrial controllers. Such controllers are easier to program and have reduced installation costs because of their use of a high-speed serial communication network eliminating long runs of point-to-point wiring.
Unfortunately, high-speed serial communication networks commonly used in industrial control are not sufficiently reliable for safety systems. For this reason, efforts have been undertaken to develop a “safety network” being a high-speed serial communication network providing greater certainty in the transmission of data. Currently proposed safety networks are incompatible with the protocols widely used in industrial control. Accordingly, if these new safety networks are adopted, existing industrial controller hardware and standard technologies may be unusable, imposing high costs on existing and new factories. Such costs may detrimentally postpone wide scale adoption of advanced safety technology.
Furthermore, conventional safety systems transmit redundant messages on two separate hardware paths so as to reduce the problem of hardware failure. If data is being transmitted across networks utilizing different communication protocol, a bridge must be implemented in order to allow the two communication between the networks. In order to make the bridge conform with the requirements of the safety system, it must have to separate hardware paths to process the redundant messages. Conventional bridges include only a single hardware path, and their modification to include a second hardware path is expensive and inefficient. If a conventional bridge with a single hardware path is installed, and one of the components fails, the redundant messages will still match (assuming the failure is persistent) when they are subsequently compared, and the error would go undetected.
What is needed is a safety network that is compatible with conventional industrial controller networks and components. Ideally such a safety network would work with a wide variety of different standard communication protocols and would allow the mixing of standard industrial control components and safety system components without compromising reliability. Additionally, it is desirable to implement a standard bridge having a hardware path that is operable with the redundant messages and that will allow the detection of any errors that reside in the circuitry of the bridge.